This invention relates generally to medical imaging methods. More particularly, the invention relates to image data acquisition for Magnetic Resonance (MR) angiographyy using a Magnetic Resonance Imaging (MRI) system.
MR coronary angiography offers the potential for a totally noninvasive exam which could detect and characterize lesions in the coronary arteries, thus allowing the avoidance of diagnostic X-ray catheterization in a large number of patients who currently receive this exam. In order to avoid blurring and ghosting in the MR images due to respiratory motion, Echocardiogram (ECG)-gated MR coronary angiography has typically been performed using either breath-held two-dimensional (2D) techniques or respiratory-gated three-dimensional (3D) techniques. Repeated breath holding is not feasible in a significant percentage of coronary patients and often leads to misregistration artifacts. Respiratory gating has been performed using navigator-echo gating techniques, where, interleaved with the imaging acquisition, a column of magnetization is excited running through the diaphragm, and data are acquired in the presence of a readout gradient oriented along the column axis. A one-dimensional Fourier transform of the data yields the position of the diaphragm as a function of time, which is then used either to trigger the acquisition of new coronary imaging data, or to reacquire data collected while the heart was mispositioned, or to acquire data with slice and/or phase shifts which track the motion of the heart. This technique, however, has not provided a robust method that works over a range of different breathing patterns in a variety of patients. Moreover, the displacement of the coronary arteries with respiration has been shown to differ from the displacement of the diaphragm, by varying amounts in different subjects, making the diaphragm a poor marker for cardiac positional information. Spiral navigation techniques replace the column excitation with a spiral trajectory, and have been used to detect translations and rotations in the collected images in the context of head imaging. Corrections can then be applied to the images by adjusting the phase or center position of the k-space, or time-domain, images. However, these techniques are not readily adapted to the heart, where significant through-plane motion and deformation are occurring as well as in-plane motion. An adaptive averaging technique, where frames of data where the coronary artery is visible are averaged after translation using cross-correlation algorithms, provides coronary imaging capabilities without breath holding or ECG gating, however image resolution is not optimal with this technique. Therefore a robust free-breathing technique is needed for coronary MRI which directly monitors the position of the vessels without extended breath-holding, and which produces high-resolution images.